Aftertreatment catalyst degradation compensation

ABSTRACT

One embodiment is a method including providing an aftertreatment catalyst disposed in an exhaust stream of an internal combustion engine and a reductant that reacts with an amount of NOx in the exhaust stream in the presence of the aftertreatment catalyst. The method includes interpreting a catalyst degradation value corresponding to the aftertreatment catalyst, and interpreting a nominal reductant value. The method further includes determining an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value, and injecting the reductant into the exhaust stream at a rate based on the adjusted reductant value.

RELATED APPLICATIONS

The present application is a U.S. Patent Application of PCT/US2010/024954 entitled “Aftertreatment Catalyst Degradation Compensation,” filed Feb. 22, 2010, which claims priority to U.S. Provisional Patent Application 61/154,561, filed Feb. 23, 2009, both of which are incorporated herein by reference.

TECHNICAL FIELD

The technical field generally relates to internal combustion engine aftertreatment systems.

BACKGROUND

Many current powertrain systems include an aftertreatment system in the exhaust of internal combustion engines to meet emissions regulations or to reduce emissions of undesirable exhaust gas constituents. A variety of aftertreatment systems include one or more catalytic components that experience degradation and/or reduced efficiency over time. Efficiency reductions can affect the conversion capability of the catalyst, and can also affect the storage capacity of the catalyst as an adsorption device. Degradation of a catalyst introduces challenges into the system to continue to meet emissions targets and in some cases to continue to meet target levels of undesirable constituents in the exhaust. In some systems, a system response to continue to meet emissions may increase the amount of undesirable constituents in the exhaust. In some systems, the appropriate system response to a catalyst degradation may be variable depending upon the current operating conditions. Therefore, further improvements in this area of technology are desirable.

SUMMARY

One embodiment is a unique method for compensating for catalyst degradation in an aftertreatment system. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for compensating for catalyst degradation.

FIG. 2 is a schematic diagram of an apparatus for compensating for catalyst degradation.

FIG. 3 is an illustration of a space-velocity temperature map for a non-degraded catalyst.

FIG. 4 is an illustration of a space-velocity temperature map for a degraded catalyst.

FIG. 5 is an illustration of catalyst degradation values as a function of temperature.

FIG. 6 is an alternate illustration of catalyst degradation values as a function of temperature.

FIG. 7 is a schematic flow diagram of a procedure for compensating for catalyst degradation.

FIG. 8 is a schematic flow diagram of an alternate procedure for compensating for catalyst degradation.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, any alterations and further modifications in the illustrated embodiments, and any further applications of the principles of the invention as illustrated therein as would normally occur to one skilled in the art to which the invention relates are contemplated herein.

FIG. 1 is a schematic diagram of a system 100 for compensating for catalyst degradation. The system 100 includes an aftertreatment catalyst 102 disposed in an exhaust stream 104 of an internal combustion engine 106. The aftertreatment catalyst 102 may be any type of catalyst that works in conjunction with a reductant 108, and includes at least a selective catalytic reduction (SCR) catalyst or a NO_(x) adsorption catalyst. The system 100 may further include a particulate filter (not shown) upstream or downstream of the aftertreatment catalyst 102, and a clean-up catalyst 122 downstream of the aftertreatment catalyst 102. In certain embodiments, the clean-up catalyst 122 may be an ammonia oxidation catalyst, a catalyst to oxidize unburned hydrocarbons, or to catalyze any other undesirable constituents of the exhaust stream 104.

The system 100 includes the reductant 108 that reacts with an amount of NO_(x) in the exhaust stream 104 in the presence of the aftertreatment catalyst 102. The reductant 108 includes any product known to reduce NO_(x) including at least ammonia, urea, and/or hydrocarbons. The reductant 108 may be injected at other locations in the system 100, including addition of hydrocarbons by post-injection of fuel in the engine 106. The system 100 further includes an injector 110 that injects the reductant 108 into the exhaust stream 104 at a position upstream of the aftertreatment catalyst 102. The injector 110 may be located at any position in the system 100 upstream of the aftertreatment catalyst 102, including at least a position upstream of a turbocharger 112 and a position within a cylinder of the engine 106.

In certain embodiments, the system 100 further includes a controller 120 structured to perform certain operations to compensate for catalyst degradation. In certain embodiments, the controller 120 forms a portion of a processing subsystem including one or more computing devices having memory, processing, and communication hardware. The controller 120 may be a single device or a distributed device, and the functions of the controller 120 may be performed by hardware or software.

In certain embodiments, the controller 120 includes one or more modules structured to functionally execute the operations of the controller. The description herein including modules emphasizes the structural independence of the aspects of the controller 120, and illustrates one grouping of operations and responsibilities of the controller 120. Other groupings that execute similar overall operations are understood within the scope of the present application. Modules may be implemented in hardware and/or software on computer readable medium, and modules may be distributed across various hardware or software components. More specific descriptions of certain embodiments of controller 120 operations are included in the section referencing FIG. 2. The controller includes a degradation module, an injection amount module, a degradation compensation module, and/or an injection control module.

In certain embodiments, the degradation module interprets a catalyst degradation value corresponding to the aftertreatment catalyst 102, the injection amount module interprets a nominal reductant value comprising a nominal reductant to NO_(x) ratio, and the degradation compensation module determines an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value. In certain further embodiments, the controller 120 includes an injection control module that provides an injection rate signal based on the adjusted reductant value. The injector 110 injects the reductant 108 into the exhaust stream 104 at a position upstream of the aftertreatment catalyst 102 in response to the injection rate signal.

FIG. 2 is a schematic diagram of an apparatus 200 for compensating for catalyst degradation. The apparatus 200 may be a portion of a processing subsystem including a controller 120. The controller 120 includes a degradation module 202, an injection amount module 204, and a degradation compensation module 206.

The degradation module 202 interprets a catalyst degradation value 210 corresponding to the aftertreatment catalyst 102. Interpreting the catalyst degradation value 210 includes reading the catalyst degradation value 210 from a memory location, from a user input, over a datalink, and/or determining the catalyst degradation value 210 based on other parameters in the system 100.

In certain embodiments, the catalyst degradation value 210 includes a NO_(x) conversion efficiency value 218 and/or a reductant storage value 220. For example, the catalyst degradation value 210 may include a description of a NO_(x) conversion efficiency at a standard set of conditions based on the current level of catalyst degradation, and/or may include a description of a reductant storage amount (e.g. maximum storage value). The reductant storage amount includes a storage description for the reductant 108 (e.g. ammonia) as the reductant 108 is stored on the aftertreatment catalyst 102, and may further include a description of adsorption and desorption dynamics of the reductant 108. For example, the reductant 108 may be urea, and the catalyst degradation value 210 may include a reductant storage value 220 that describes an amount of ammonia that can be stored on the aftertreatment catalyst 102 at a standard set of conditions.

The catalyst degradation value 210 may be a function of a temperature of the aftertreatment catalyst 102 and/or a function of a space velocity of the exhaust stream 104 in the aftertreatment catalyst 102. For example, the catalyst degradation value 210 may include a description of a maximum amount of ammonia storage on the aftertreatment catalyst 102 across a range of operating temperatures. In another example, the catalyst degradation value 210 may include a description of a NO_(x) conversion efficiency (i.e. moles of NO_(x) converted for each mole of the limiting reactant available—either NO_(x) or reductant) across a range of operating temperatures. In another example, the catalyst degradation value 210 may include a description of a NO_(x) conversion efficiency across a range of space velocity values. In yet another example, the catalyst degradation value 210 may include a two-dimensional description of a NO_(x) conversion efficiency across a range of space velocity values and a range of temperatures. In another example, the catalyst degradation value 210 includes a description of a reductant storage amount based upon an amount of reductant present in the exhaust stream 104—as the dynamic equilibrium of the system allows for increased reductant storage in some catalyst systems where excess reductant is present. All examples are provided for purposes of illustration only, and are not considered to be limiting.

In some embodiments, the degradation module 202 further defines a first compensation region 224, a second compensation region 226, and an efficient catalyst region 228 on a space-velocity temperature map 222 in response to the catalyst degradation value 210. The degradation module 202 further determines the adjusted reductant value 208 as a value higher than the nominal reductant value 212 in the first compensation region 224 and as a value lower than the nominal reductant value 212 in the second compensation region 226. In certain embodiments, the degradation module 202 does not adjust the nominal reductant value 212 when operating conditions are in the efficient catalyst region 228.

The injection amount module 204 interprets a nominal reductant value 212 comprising a nominal reductant to NO_(x) ratio. The nominal reductant to NO_(x) ratio may be a molar ratio of reductant to NO_(x). Interpreting the nominal reductant value 212 includes reading the nominal reductant value 212 from a memory location, from a user input, over a datalink, and/or determining the nominal reductant value 212 based on other parameters in the system 100. The nominal reductant value 212 is a value of reductant to NO_(x) ratio that would be utilized in the absence of catalyst degradation compensation, and is not necessarily a single or static value. The nominal reductant value 212 may be a dynamic value based on various parameters in the system 100 including, but not limited to, an NO:N0₂ ratio in the exhaust stream, an amount of NO_(x) in the exhaust stream, the present temperature of the aftertreatment catalyst 102 and/or exhaust stream 104, and the current space velocity of the aftertreatment catalyst 102 based on the present exhaust stream 104 flow rate and amount of catalyst in the aftertreatment catalyst 102.

The degradation compensation module 206 determines an adjusted reductant value 208 in response to the catalyst degradation value 210 and the nominal reductant value 212. In certain embodiments, the adjusted reductant value 208 is determined as a value to achieve similar results to the nominal reductant value 212 in a non-degraded catalyst, and/or a value to achieve at least a minimal set of results for emissions or other purposes. In one example, the degradation compensation module 206 determines the adjusted reductant value 208 as a value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value 212. In another example, the degradation compensation module 206 determines the adjusted reductant value 208 as a value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value limited by an ammonia slip amount 230. In another example, the degradation compensation module 206 determines the adjusted reductant value 208 as a value that provides at least a minimum NO_(x) reduction amount 232. In another example, the degradation compensation module 206 determines the adjusted reductant value 208 as a value that does not exceed an ammonia slip amount 230. Examples are provided for illustration only and are not considered limiting.

In certain further embodiments, interpreting a nominal reductant value includes determining a nominal reductant to NO_(x) ratio, and determining an adjusted reductant value includes determining an adjusted reductant to NO_(x) ratio. In certain further embodiments, the adjusted reductant value is an adjusted reductant to NO_(x) ratio. The adjusted reductant to NO_(x) ratio may be a value between 0.5 and 1.6. In certain embodiments, the adjusted reductant to NO_(x) ratio is a value greater than 0.3, a value less than 3, and/or a value less than 5. Generally, the NO_(x) conversion will not be greater than the NO_(x) ratio, so a NO_(x) ratio of 0.3 provides about 30% maximum conversion of NO_(x) (which may not be fully achieved). Some benefits in NO_(x) conversion continue with ratios of 3:1, 5:1, or even greater. In some systems, where an effective ammonia oxidation catalyst 122 is present, or where avoidance of NO_(x) is more important than slip of ammonia, higher reductant to NO_(x) ratios may be indicated.

In certain embodiments, the controller 120 includes an injection control module 214 that provides an injection rate signal 216 based on the adjusted reductant value 208. The injection rate signal 216 may be a parameter determined from the adjusted reductant value 208, and may be otherwise adjusted due to injector dynamics, unit conversions, system limitations, or for other adjustments known in the art.

FIG. 3 is an illustration of a space-velocity temperature map for a non-degraded catalyst. The regions 302, 304, 306, 308, 310 illustrated are regions of decreasing NO_(x) conversion efficiency, in the order listed where the region 302 is a highest efficiency and the region 310 is a lowest efficiency. Referring to FIG. 4, an illustration of a space-velocity temperature map for a degraded catalyst is illustrated. The region 406 is an illustration of the region 302 of highest efficiency from the non-degraded catalyst of FIG. 3. In the example shown, the highest efficiency region 302 occurs at a lower temperature in the degraded catalyst than in the non-degraded catalyst. A first compensation region 224 is illustrated and a second compensation region 226 is illustrated. The adjusted reductant value 208 may be higher than the nominal reductant value 212 in the first compensation region 224, and the adjusted reductant value 208 may be lower than the nominal reductant value 212 in the second compensation region 226. The responses illustrated are for example purposes only, and the emissions and degraded catalyst behaviors for a given embodiment of the system 100 determine specific responses for the system 100.

The determination of efficiency contours 302, 304, 306, 308, 310, the number of contours, and the thresholds for contours are mechanical steps for one of skill in the art with the benefit of the disclosures herein. The region 302 of FIG. 4 may be an efficient catalyst region 228, and in certain embodiments within the efficient catalyst region the adjusted reductant value 208 may be equivalent to the nominal reductant value 212. The positioning of the efficient catalyst region 228, where present, is a decision based upon the catalyst efficiencies of the system 100 that allow the degraded catalyst to operate similarly to a non-degraded catalyst and still meet design criteria. In certain embodiments, the positioning of the efficient catalyst region 228 may significantly differ from the region 302.

FIG. 5 is an illustration of catalyst degradation values 210 as a function of temperature. The curve 502 illustrates a NO_(x) conversion efficiency at a first relatively low temperature and the curve 504 illustrates a NO_(x) conversion efficiency at a second relatively high temperature over a range of degradation values 210. The units of the degradation value 210 scale may be of any available degradation description, including a damage index, an amount of time spent above a threshold temperature, a number of damage events that have occurred (for example a number of high temperature regeneration events), or any other parameter known in the art. In one embodiment, a catalyst is exposed to an aging or damage process, and the NO_(x) conversion efficiency for the catalyst is tracked with the damage to build curves 502, 504 such as those illustrated in FIG. 5. In certain embodiments, the degradation compensation module 206 utilizes curves such as those illustrated in FIG. 5 to determine the adjusted reductant value 208 based on the nominal reductant value 212.

FIG. 6 is an alternate illustration of catalyst degradation values 210 as a function of temperature. In the illustration of FIG. 6, a reductant to NO_(x) ratio is plotted against temperature for a plurality of catalyst degradation values 210. In one example, the curve 602 illustrates the nominal reductant value 212 for a non-degraded catalyst, and the curves 604, 606, 608 illustrate adjusted reductant values 208 for the catalyst at various levels of degradation. In one example, the curve 604 represents a catalyst with 25 hours spent over a threshold temperature, the curve 606 represents a catalyst with 100 hours spent over the threshold temperature, and the curve 608 represents a catalyst with 200 hours spent over the threshold temperature. In certain embodiments, the degradation compensation module 206 utilizes curves such as those illustrated in FIG. 6 to determine the adjusted reductant value 208 based on the nominal reductant value 212. In one example, a number of curves 602, 604, 606, 608 are stored on the controller 120 and the degradation compensation module 206 interpolates between curves 602, 604, 606, 608 based upon the catalyst degradation value 210 compared to the catalyst degradation values associated with the curves 602, 604, 606, 608.

The schematic flow diagrams of FIGS. 7 and 8, and related description which follows provides an illustrative embodiment of performing procedures for degraded catalyst compensation. Operations illustrated are understood to be exemplary only, and operations may be combined or divided, and added or removed, as well as re-ordered in whole or part, unless stated explicitly to the contrary herein.

FIG. 7 is a schematic flow diagram of a procedure 700 for compensating for catalyst degradation. The procedure 700 includes an operation 702 to provide an aftertreatment catalyst disposed in an exhaust stream of an internal combustion engine and an operation 704 to provide a reductant that reacts with an amount of NO_(x) in the exhaust stream in the 5 presence of the aftertreatment catalyst. The procedure 700 further includes an operation 706 to interpret a catalyst degradation value corresponding to the aftertreatment catalyst, and an operation 708 to interpret a nominal reductant value. The procedure 700 further includes an operation 710 to determine an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value. The procedure 700 further 10 includes an operation 712 to inject the reductant into the exhaust stream at a rate based on the adjusted reductant value.

FIG. 8 is a schematic flow diagram of an alternate procedure 800 for compensating for catalyst degradation. The procedure 800 includes an operation 802 to interpret a catalyst degradation value corresponding to an aftertreatment catalyst, and an operation 804 to interpret a nominal reductant value comprising a nominal reductant to NO_(x) ratio.

The procedure 800 further includes an operation 806 to determine an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value. The procedure 800 further includes an operation 818 to provide an injection rate signal based on the adjusted reductant value, and an operation 820 to inject an amount of reductant into an exhaust stream in response to the injection rate signal.

In certain embodiments, the procedure 800 includes an operation 808 to select an adjustment method for determining the adjusted reductant value. An adjustment method “A” includes an operation 810 to determine an adjusted reductant value that provides a NOx reduction amount achievable by a non-degraded catalyst with the nominal reductant value. An adjustment method “B” includes an operation 812 to determine an adjusted reductant value that provides a NOx reduction amount achievable by a non-degraded catalyst with the nominal reductant value limited by an ammonia slip amount. An adjustment method “C” includes an operation 814 to determine an adjusted reductant value that provides at least a minimum NOx reduction amount. An adjustment method “D” includes an operation 816 to determine an adjusted reductant value that provides an adjusted reductant value that does not exceed an ammonia slip amount.

As is evident from the figures and text presented above, a variety of embodiments according to the present invention are contemplated.

One exemplary embodiment is a method including providing an aftertreatment catalyst disposed in an exhaust stream of an internal combustion engine and providing a reductant that reacts with an amount of NO_(x) in the exhaust stream in the presence of the aftertreatment catalyst. The method further includes interpreting a catalyst degradation value corresponding to the aftertreatment catalyst, interpreting a nominal reductant value, and determining an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value. The method further includes injecting the reductant into the exhaust stream at a rate based on the adjusted reductant value.

In a further embodiment, the reductant includes urea, ammonia, and/or a hydrocarbon. The catalyst degradation value includes a NO_(x) conversion efficiency value and/or a reductant storage value. In certain embodiments, the catalyst degradation value further includes a function of a temperature of the aftertreatment catalyst, and/or a function of a space velocity of the exhaust stream in the aftertreatment catalyst. In some embodiments, the nominal reductant value includes a molar ratio of the reductant to the amount of NO_(x). In certain further embodiments, interpreting a nominal reductant value includes determining a nominal reductant to NO_(x) ratio, and determining an adjusted reductant value includes determining an adjusted reductant to NO_(x) ratio. In certain further embodiments, the adjusted reductant value is an adjusted reductant to NO_(x) ratio, and is a value between 0.5 and 1.6, a value greater than 0.3, a value less than 3, and/or a value less than 5.

Another exemplary embodiment is a system including an aftertreatment catalyst disposed in an exhaust stream of an internal combustion engine, a reductant that reacts with an amount of NO_(x) in the exhaust stream in the presence of the aftertreatment catalyst, and an injector structured to inject the reductant into the exhaust stream at a position upstream of the aftertreatment catalyst. The system may include a controller having modules structured to functionally execute operations to compensate for catalyst degradation. The controller includes a degradation module, an injection amount module, and a degradation compensation module.

The degradation module interprets a catalyst degradation value corresponding to the aftertreatment catalyst, the injection amount module interprets a nominal reductant value comprising a nominal reductant to NO_(x) ratio, and the degradation compensation module determines an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value. In certain embodiments, the controller includes an injection control module that provides an injection rate signal based on the adjusted reductant value. The system further includes an injector that injects the reductant into the exhaust stream at a position upstream of the aftertreatment catalyst in response to the injection rate signal.

In certain embodiments, the catalyst degradation value includes a NO_(x) conversion efficiency value and/or a reductant storage value. The catalyst degradation value may be a function of a temperature of the aftertreatment catalyst and/or a function of a space velocity of the exhaust stream in the aftertreatment catalyst. In some embodiments, the degradation module further defines a first compensation region, a second compensation region, and an efficient catalyst region on a space-velocity temperature map in response to the catalyst degradation value, and further determines the adjusted reductant value as a value higher than the nominal reductant value in the first compensation region and as a value lower than the nominal reductant value in the second compensation region.

In some embodiments the degradation compensation module further determines the adjusted reductant value according to at least one of the following schemes: determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value, determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value limited by an ammonia slip amount, determining an adjusted reductant value that provides at least a minimum NO_(x) reduction amount, and determining an adjusted reductant value that does not exceed an ammonia slip amount.

Yet another exemplary embodiment is an apparatus including a degradation module that interprets a catalyst degradation value corresponding to an aftertreatment catalyst, an injection amount module that interprets a nominal reductant value including a nominal reductant to NO_(x) ratio, a degradation compensation module that determines an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value, and an injection control module that provides an injection rate signal based on the adjusted reductant value. The degradation module may further define a first compensation region, a second compensation region, and an efficient catalyst region on a space-velocity temperature map in response to the catalyst degradation value. In certain embodiments, the degradation compensation module further determines the adjusted reductant value as a value higher than the nominal reductant value in the first compensation region, and determines the adjusted reductant value as a value lower than the nominal reductant value in the second compensation region.

One exemplary embodiment is a method that includes interpreting a catalyst degradation value corresponding to an aftertreatment catalyst, interpreting a nominal reductant value comprising a nominal reductant to NO_(x) ratio, determining an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value, providing an injection rate signal based on the adjusted reductant value, and injecting an amount of reductant into an exhaust stream in response to the injection rate signal. In certain embodiments, the method further includes determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value, determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value limited by an ammonia slip amount, determining an adjusted reductant value that provides at least a minimum NO_(x) reduction amount, and determining an adjusted reductant value that does not exceed an ammonia slip amount.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain exemplary embodiments have been shown and described and that all changes and modifications that come within the spirit of the inventions are desired to be protected. It should be understood that while the use of words such as preferable, preferably, preferred or more preferred utilized in the description above indicate that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the invention, the scope being defined by the claims that follow. In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary. 

What is claimed is:
 1. A method, comprising: providing an aftertreatment catalyst disposed in an exhaust stream of an internal combustion engine; providing a reductant that reacts with an amount of NO_(x) in the exhaust stream in the presence of the aftertreatment catalyst; interpreting a catalyst degradation value corresponding to the aftertreatment catalyst; interpreting a nominal reductant value; determining an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value; and injecting the reductant into the exhaust stream at a rate based on the adjusted reductant value.
 2. The method of claim 1, wherein the reductant comprises a reductant selected from the group consisting of urea, ammonia, and a hydrocarbon.
 3. The method of claim 1, wherein the catalyst degradation value comprises at least one value selected from the values consisting of a NO_(x) conversion efficiency value and a reductant storage value.
 4. The method of claim 3, wherein the catalyst degradation value further comprises a function of a temperature of the aftertreatment catalyst.
 5. The method of claim 3, wherein the catalyst degradation value further comprises a function of a space velocity of the exhaust stream in the aftertreatment catalyst.
 6. The method of claim 1, wherein the nominal reductant value comprises a molar ratio of the reductant to the amount of NO_(x).
 7. The method of claim 1, wherein: the catalyst degradation value comprises at least one value selected from the values consisting of a NO_(x) conversion efficiency value and a reductant storage value; the catalyst degradation value further comprises a function of a temperature of the aftertreatment catalyst and a space velocity of the exhaust stream in the aftertreatment catalyst; wherein the interpreting a nominal reductant value comprises determining a nominal reductant to NO_(x) ratio; and wherein the determining an adjusted reductant value comprises determining an adjusted reductant to NO_(x) ratio.
 8. The method of claim 1, wherein: the catalyst degradation value comprises at least one value selected from the values consisting of a NO_(x) conversion efficiency value and a reductant storage value; the catalyst degradation value further comprises a function of a temperature of the aftertreatment catalyst; wherein the interpreting a nominal reductant value comprises determining a nominal reductant to NO_(x) ratio; and wherein the determining an adjusted reductant value comprises determining an adjusted reductant to NO_(x) ratio.
 9. The method of claim 8, wherein the adjusted reductant value comprises a value between 0.5 and 1.6, inclusive.
 10. The method of claim 8, wherein the adjusted reductant value comprises a value greater than 0.3, inclusive.
 11. The method of claim 8, wherein the adjusted reductant value comprises a value less than 3, inclusive.
 12. The method of claim 8, wherein the adjusted reductant value comprises a value less than 5, inclusive.
 13. A system, comprising: an aftertreatment catalyst disposed in an exhaust stream of an internal combustion engine; a reductant that reacts with an amount of NO_(x) in the exhaust stream in the presence of the aftertreatment catalyst; an injector structured to inject the reductant into the exhaust stream at a position upstream of the aftertreatment catalyst; a degradation module structured to interpret a catalyst degradation value corresponding to the aftertreatment catalyst; an injection amount module structured to interpret a nominal reductant value comprising a nominal reductant to NO_(x) ratio; a degradation compensation module structured to determine an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value; and an injection control module structured to provide an injection rate signal based on the adjusted reductant value; and an injector structured to inject the reductant into the exhaust stream at a position upstream of the aftertreatment catalyst in response to the injection rate signal.
 14. The system of claim 13, wherein the catalyst degradation value comprises at least one value selected from the values consisting of a NOx conversion efficiency value and a reductant storage value.
 15. The system of claim 14, wherein the catalyst degradation value further comprises a function of a temperature of the aftertreatment catalyst.
 16. The system of claim 14, wherein the catalyst degradation value further comprises a function of a space velocity of the exhaust stream in the aftertreatment catalyst.
 17. The system of claim 13, wherein the degradation module is further structured to define a first compensation region, a second compensation region, and an efficient catalyst region on a space-velocity temperature map in response to the catalyst degradation value, and wherein the degradation compensation module is further structured to: determine the adjusted reductant value as a value higher than the nominal reductant value in the first compensation region; and determine the adjusted reductant value as a value lower than the nominal reductant value in the second compensation region.
 18. The system of claim 13, wherein the degradation compensation module is further structured to determine the adjusted reductant value according to one of the determination schemes selected from the schemes consisting of: determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value; determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value limited by an ammonia slip amount; determining an adjusted reductant value that provides at least a minimum NO_(x) reduction amount; and determining an adjusted reductant value that does not exceed an ammonia slip amount.
 19. An apparatus, comprising: a degradation module structured to interpret a catalyst degradation value corresponding to an aftertreatment catalyst; an injection amount module structured to interpret a nominal reductant value comprising a nominal reductant to NO_(x) ratio; a degradation compensation module structured to determine an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value; and an injection control module structured to provide an injection rate signal based on the adjusted reductant value.
 20. The apparatus of claim 19, wherein the degradation module is further structured to define a first compensation region, a second compensation region, and an efficient catalyst region on a space-velocity temperature map in response to the catalyst degradation value, and wherein the degradation compensation module is further structured to: determine the adjusted reductant value as a value higher than the nominal reductant value in the first compensation region; and determine the adjusted reductant value as a value lower than the nominal reductant value in the second compensation region.
 21. The apparatus of claim 19, wherein the degradation compensation module is further structured to determine the adjusted reductant value according to one of the determination schemes selected from the schemes consisting of: determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value; determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value limited by an ammonia slip amount; determining an adjusted reductant value that provides at least a minimum NO_(x) reduction amount; and determining an adjusted reductant value that does not exceed an ammonia slip amount.
 22. A method, comprising: interpreting a catalyst degradation value corresponding to an aftertreatment catalyst; interpreting a nominal reductant value comprising a nominal reductant to NO_(x) ratio; determining an adjusted reductant value in response to the catalyst degradation value and the nominal reductant value; providing an injection rate signal based on the adjusted reductant value; and injecting an amount of reductant into an exhaust stream in response to the injection rate signal.
 23. The method of claim 22, wherein determining the adjusted reductant value comprises determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value.
 24. The method of claim 22, wherein determining the adjusted reductant value comprises determining an adjusted reductant value that provides a NO_(x) reduction amount achievable by a non-degraded catalyst with the nominal reductant value limited by an ammonia slip amount.
 25. The method of claim 22, wherein determining the adjusted reductant value comprises determining an adjusted reductant value that provides at least a minimum NO_(x) reduction amount.
 26. The method of claim 22, wherein determining the adjusted reductant value comprises determining an adjusted reductant value that does not exceed an ammonia slip amount. 